Never seen any direct drive pump attached with a screw unless it is an aftermarket pump, OEM pumps are held on only with the two clamps, if it will not pull off the pump bearings are stuck to the shaft of the motor because of rust swelling the motor shaft, this happens when the pump has been leaking for months. About down to smashing the pump off and hope you can sand the motor shaft back to close to normal so it will accept a new pump.